System for measuring the luminance characteristics of objects, in particular objects with luminance dependent on emitting direction

ABSTRACT

A system for measuring the luminance characteristics of objects, particularly objects with luminance dependent on the emission direction. 
     This system includes an image sensor ( 8 ) and optical means ( 16, 18, 26, 28 ) provided to form the image of the totality of an object ( 6 ) on the sensor and to select, for each point of the object, with a view to forming the corresponding image-point, those of the light rays coming from this point of the object which propagate in a way approximately parallel to the optical axis (X) of the optical means.

TECHNICAL FIELD

The present invention relates to a system for measuring the luminancecharacteristics of objects, particularly objects with luminancedependent on the emission direction.

It applies for example to projection screens, to cathode ray tubes, tolighting devices and to display screens such as liquid crystal displays,plasma displays, electroluminescent displays and microtip screens.

THE PRIOR ART

Several systems for measuring the luminance characteristics of objectsin accordance with a geometrical position so as to characterise theiruniformity and properties are already known. Such systems may be usedwith the objects given above for example.

In particular an electromechanical system is known which isdiagrammatically shown in FIG. 1.

This system includes:

a measuring instrument 2, for example a photometer, and

movement means 4 of this instrument in front of an object to be measured6 (these means 4 being symbolised by arrows in FIG. 1).

This electromechanical system allows measurements to be taken along theoptical axis X of the measuring instrument (and therefore according toan angle θ zero relative to this axis) but has however numerousdrawbacks. In particular the measurements are taken by sampling. Onlythe positions selected are measured and no information is known aboutluminance in the intermediate positions. No certainty exists as to thevalue of the luminance outside that of the points measured. Moreover,the measurements, of duration T₀, are taken in series, one after theother. If a great number N of points are to be measured in order to beable to have maximum information, the complete measurement of the objecttakes a time NXT₀.

Another measurement system is also known and this is diagrammaticallyshown in FIG. 2.

This system includes:

a matrix sensor 8 of the CCD type or similar and

a lens 10 which is between this sensor and the object 6 to be measuredand allows the image of this latter to be formed on the sensor.

Thus, at one and the same time, an image of the object to be measured isobtained on the sensor. The different points of the image correspond tothe measurements relative to the different points of the object to bemeasured.

The main advantages of this sensor system are as follows:

Measurement speed is increased. Indeed, measurement, of duration T₁,does not depend (or only slightly) on the number of points measured. Allthe information is available. There is no risk of seeing a detail of theimage evade measurement. An integration (summation) of all the valuesobtained gives with certainty a value of the luminous flux emitted bythe object.

However the system shown in FIG. 2 has a serious drawback which is showndiagrammatically by FIG. 3. The lens 10, the axis of which is denoted Xand which is conventionally used for such a system, operates at aconstant image size D₂=2d₂. An object of size D₂=2d₁ must therefore beat a distance L₁ from this lens such that:

L ₁/(2d ₁)=L ₂/(2d ₂)=K

where L₂ is the distance between the lens 10 and the sensor 8 and K is aconstant.

The object is observed along an angle θ which depends on the measuredpoint of the object (θ is counted relative to a straight line passingthrough this point and parallel to the optical axis X of the lens 10)and which, for the end points, takes a value θ_(M) (FIG. 2) littledifferent from tan θ_(M) and therefore little different from d₂/L₂ inother words from 1/(2K).

Usually K is of the order of 2-5 to 3 (which means it is necessary toplace 75 cm from the lens a 12′ (about 30 cm) diagonal screen which itis wished to measure) so that θ_(M) is of the order of 12°. Measurementsare therefore taken at a variable angle, according to the position,between 0° (measurement along the axis X) and ±12°.

This would not be a drawback if the objects measured had an emissioncharacteristic such that the luminance does not vary in accordance withthe light emission direction in other words in accordance with the angleθ.

This is not usually the case and it is clear that a system of the typeof that in FIGS. 2 and 3 does not allow the emission uniformity of anobject to be measured independently of the emission characteristic ofthat object.

DISCLOSURE OF THE INVENTION

The purpose of the present invention is to overcome this drawback.

It relates a system allowing the luminance characteristics of objects tobe accurately measured, whether or not their luminance varies inaccordance with the emission direction.

The invention combines the advantages of the system in FIG. 1 (withwhich measurements are taken at θ=0°) and those of the system in FIG. 2(with which the measurements are taken rapidly).

In an exact way, an object of the present invention is a system formeasuring the luminance characteristics of objects, this systemincluding:

an image sensor and

optical means having an optical axis and provided to form the image ofthe totality of an object on the sensor, each point of the imageallowing a measurement to be taken on a point of the object,

this system being characterised in that the optical means areadditionally provided to select, for each point of the object, with aview to forming the corresponding image-point, those of the light rayscoming from this point of the object which propagate in a wayapproximately parallel to the optical axis of the optical means.

According to a preferred embodiment of the system object of theinvention, the optical means include:

a first lens placed facing the object,

a diaphragm placed between the first lens and the sensor and able to letpass, among the light rays coming to it from the object through thefirst lens, only those which propagate from the object in a wayapproximately parallel to the optical axis of the first lens, and

auxiliary optical means placed between the diaphragm and the sensor andprovided to form, from the light rays which the diaphragm lets pass, theimage of the object in an observation plane, the sensor beingapproximately placed in this observation plane.

Preferably, the object is approximately placed in the object focal planeof the first lens and the diaphragm is approximately placed in the imagefocal plane of this first lens.

According to a first particular embodiment of the invention, theauxiliary optical means include a second lens provided to form the imageof the object in the observation plane.

According to a second particular embodiment, the auxiliary optical meansinclude:

a second lens provided to form an intermediate image of the object in anintermediate plane and

a third lens placed between the second lens and the sensor and providedto form the image of the object in the observation plane from theintermediate image and to adapt the size of the image of the object tothe size of the sensor.

The aperture of the diaphragm can be variable. Moreover, the systemobject of the invention can include an optical filtering means of thelight coming from the object.

In the case of the first particular embodiment mentioned above, thisfiltering means is preferably approximately placed in the observationplane, facing the sensor. In the case of the second particularembodiment, this filtering means is preferably approximately placed inthe intermediate plane.

In the case of one or the other of these particular embodiments, thesecond lens is preferably provided in order that the light rays whichcome from the object and which reach the optical filtering means areperpendicular to the plane where this optical filtering means islocated.

The sensor is preferably of the matrix type.

Indeed a device for surveying the luminous emission properties of alight emitting surface is known from the document FR2715470A. However,in this document, it is a question of measuring average intensity on asurface delimited by a diaphragm, by means of a single sensor. Theinformation obtained is a single quantity. The means implemented aim toobtain a uniform response over a controlled surface size.

On the contrary, in the present invention, it is a question of measuringthe luminance, point by point, of an extended object. The informationobtained is a set of quantities (cartography). The means implemented aimto make an observation at a constant angle.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thedescription of embodiment examples given below, purely indicatively andin no way restrictively, with reference to the appended drawings inwhich:

FIG. 1, already described, is a diagrammatic view of a known system formeasuring the luminance characteristics of objects,

FIG. 2, already described, is a diagrammatic view of another knownsystem for measuring such characteristics,

FIG. 3, already described, shows diagrammatically the drawbacks of thisother known system,

FIG. 4 shows diagrammatically the principle of the invention, and

FIG. 5 is a diagrammatic view of a particular embodiment of the systemobject of the invention.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

In FIG. 4 can be seen a system according to the invention, allowing theluminance characteristics of objects to be measured. This systemincludes an image sensor 8 connected to electronic means 12 intended toprocess the signals supplied by this sensor, and optical means 14allowing the image of the totality of an object 6 to be formed on thesensor 8, each point of the image allowing a measurement to be taken ona point of the object.

According to the invention the optical means 14 makes it possiblemoreover to select, for each point M of the object, with a view toforming the corresponding image-point, those of the light rays comingfrom this point M which propagate approximately along the straight linex passing through M and parallel to the optical axis X of the opticalmeans 14.

The object 6 is for example a display screen which has been switched on(or a projection screen lit by means not shown) the luminancecharacteristics of which it is desired to measure.

The sensor 8 is a sensor of the matrix type, for example a CCD sensor orsimilar, and the electronic means 12 receive the signals supplied bythis sensor and process them in a known way in order to determine theluminance characteristics of the object.

FIG. 5 is a diagrammatic view of a particular embodiment of the systemobject of the invention.

In the example in FIG. 5, the optical means 14 of FIG. 4 include a firstlens 16 the optical axis of which constitutes the axis X, a diaphragm 18and auxiliary optical means which will be discussed below. The lens 16is placed between the diaphragm 18 and the object 6. As for thediaphragm 18, this is placed between the lens 16 and the sensor 8 andprovided so as to let pass, among the light rays coming to it from theobject 6 through the lens 16, only those which propagate from the objectin a way approximately parallel to the axis X in other wordsapproximately along straight lines parallel to X such as the straightline x passing through the point M of the object.

The surface of the object 6 is approximately placed in the object focalplane OFP of the lens 16 and the diaphragm 18 is approximately placed inthe image focal plane IFP of the lens 16.

The aperture of the diaphragm 18 allows the angle of the scanning beamssuch as the light beam 22 coming from the object to be controlled.

The diaphragm 18 may be fitted with means not shown allowing itsaperture to be modified in order to modify the scanning beam angle.

The auxiliary optical means are placed between the diaphragm and thesensor 8 and are provided to form, from the light rays which thediaphragm lets pass, the image of the object 6 in an observation planewhere the sensor 8 or more exactly the input face of this sensor 8 isapproximately placed.

In a first example the auxiliary optical means include simply a lens 26provided to form the image of the object in a plane P₁ which thenconstitutes the observation plane where the sensor 8 is approximatelyplaced.

In a second example (FIG. 5) the auxiliary optical means include thislens 26 provided to form an intermediate image of the object 6 in theplane P₁, which then constitutes an intermediate plane, and additionallyanother lens 28 placed between the lens 26 and the sensor. This otherlens 28 constitutes a relay lens or transport lens which allows theimage of the object to be formed, from the intermediate image, in aplane P₂ then constituting the observation plane where the sensor 8 isapproximately placed. This lens 28 is intended to “set to scale”, in theplane P₂, the image obtained in the plane P1, i.e. to adapt the size ofthis image to the size of the sensor.

In the system in FIG. 5, an optical filter 30 can also be used. This maybe for example a spectral response correction filter which selectswavelengths so as to reproduce a particular response (for exampleresponse by the eye) or a polarising filter to select a particularpolarisation or a filter which absorbs the light in a variable way.

The filter 30 may be placed anywhere between the object 6 and the sensor8 but, in order to have reasonable size, it is to advantage placed atthe level of the plane P₁ or in a plane parallel to this plane P₁ andnear to it. Clearly, when the sensor is placed at the level of plane P₁,the filter 30 is placed facing the sensor near to it, between this planeP₁ and the lens 26.

In the event of the filter 30 being used, the lens 26 is to advantageoptimised so that the light rays which come from the object and whichreach this filter 30 (after having passed through the lens 26) areperpendicular to the plane where the filter 30 is placed (and aretherefore directed along a perpendicular y to the plane P₁ when thefilter is located in this plane P₁) so as to avoid the transmission ofthe system being dependent on the position observed on the object.

It will be noticed that the aperture of the diaphragm 18 is centred onthe optical axis X common to the lenses 16, 26 and 28 (when the latteris used).

A system of the type of the one in FIG. 5 meets all the desiredcriteria:

measurements perpendicularly to the object whatever the measured pointof the object,

speed of measurement (collective measurement)

exhaustive measurement (all the points of the object are measured).

It is also appropriate to note that an increased facility of use isobtained by means of the good depth of field of a system according tothe invention, of the type of the one in FIG. 5, due to its design. Thisproperty allows excellent tolerance of any focusing defect, which is notthe case with known systems.

What is claimed is:
 1. A system for measuring point by point theluminance characteristics of an extended object, this system including:an image sensor (8) and optical means (14) having an optical axis (X)and provided to form a image of the totality of the object (6) on thesensor, each point of the image allowing a measurement to be taken on apoint of the object, this system being characterized in that the opticalmeans (14) are additionally provided to select, for each point of theobject, with a view to forming the corresponding image-point, those ofthe light rays coming from this point of the object which propagate in away approximately parallel to the optical axis of the optical means, theobservation of the object being effected at a constant angle.
 2. Asystem according to claim 1, wherein the optical means (14) include: afirst lens (16) placed facing the object, a diaphragm (18) placedbetween the first lens and the sensor and able to let pass, among thelight rays coming to it from the object through the first lens, onlythose which propagate from the object in a way approximately parallel tothe optical axis (X) of the first lens (16), and auxiliary optical means(26, 26-28) placed between the diaphragm and the sensor and provided toform, from the light rays which the diaphragm lets pass, the image ofthe object in an observation plane (P₁;P₂), the sensor beingapproximately placed in this observation plane.
 3. A system according toclaim 2, wherein the object (6) is approximately placed in the objectfocal plane (OFP) of the first lens (16) and the diaphragm (18) isapproximately placed in the image focal plane (IFP) of this first lens(16).
 4. A system according to claim 2, wherein the auxiliary opticalmeans include a second lens (26) provided to form this image of theobject in the observation plane (P₁).
 5. A system according to claim 2,wherein the auxiliary optical means include: a second lens (26) providedto form an intermediate image of the object in an intermediate plane(P₁) and a third lens (28) placed between the second lens and the sensorand provided to form the image of the object in the observation plane(P₂) from the intermediate image and to adapt the size of the image ofthe object to the size of the sensor.
 6. A system according to claim 2wherein the aperture of the diaphragm (18) is variable.
 7. A systemaccording to claim 1, including additionally an optical filtering means(30) of the light coming from the object.
 8. A system according to claim4, including additionally an optical filtering means (30) of the lightcoming from the object, this filtering means being placed approximatelyin the observation plane (P₁), facing the sensor.
 9. A system accordingto claim 5, including additionally an optical filtering means (30) ofthe light coming from the object, this filtering means being placedapproximately in the intermediate plane (P₁).
 10. A system according toclaim 7, wherein the second lens (26) is provided in order that thelight rays which come from the object and which reach the opticalfiltering means (30) are perpendicular to the plane where this opticalfiltering means is located.
 11. A system according to claim 1, whereinthe sensor (8) is of the matrix type.
 12. A system according to claim 1,including additionally electronic means (12) for processing the signalssupplied by the sensor (8) to determine the luminance characteristics ofthe object (6).
 13. A system according to claim 8, wherein the secondlens is provided in order that the light rays which come from the objectand which reach the optical filtering means are perpendicular to theplane in which the optical filtering means is located.